The present invention relates to an optical device that is suitable for entrance and emission of light in multiple modes.
Optical fibers used in optical fiber communication systems that are generally and widely used have a structure in which the outer circumference of a single core is surrounded by cladding, and information is transmitted as a result of propagation of optical signals through the core. In recent years the amount of transmitted information has been drastically increased with the widespread use of optical fiber communication systems.
To realize the increase in the transmission capacity of such an optical fiber communication system, it is known to use a multicore fiber in which the outer circumference of multiple cores is surrounded by a single cladding to transmit multiple signals using light rays propagating through the respective cores.
An example of optical devices that causes light enter a multicore fiber and emits light therefrom is disclosed in Patent Document 1 mentioned below. This optical device is formed by integrating and stretching a single-core optical fiber in each of multiple holes formed in a capillary, the optical fibers being tapered from one ends to the other ends thereof. Each of the optical fibers has a double structure in which a core includes an inner core and outer cores surrounding the inner core without any gap therebetween. The inner core has a higher refractive index than the outer cores, and cladding has a lower refractive index than the outer cores. At the aforementioned one end that is not tapered, the diameter of the inner core and the difference in refractive index between the inner core and the outer cores are set so that single-mode light easily propagates through the inner core. At the aforementioned other end that is tapered, the outer diameter of the outer cores and the difference in refractive index between the outer cores and the cladding are set so that light diffuses to the outer cores and single-mode light easily propagates through the core combining the inner core and the outer cores.
In this optical device, the single-mode light propagating from one end to the other of each optical fiber propagates through the inner core at the one end, but light leakage from the inner core to the outer cores is increased as the diameter of the inner core becomes smaller than a predetermined diameter with the tapering. Thus, at the part where the diameter of the inner core is smaller than the predetermined diameter, light diffuses to the outer cores and propagates through the core combining the inner core and the outer cores. In this optical device, variation in the mode field diameter (MFD) of light propagating through each optical fiber is thus suppressed although the distance between optical fibers is small.    [Patent Document 1] JP-A-2013-522677